1. Field of the Invention
The present invention relates to an electric motor.
2. Description of the Related Art
There is known a permanent magnet synchronous motor that enables variable speed operation over a wide range from low speed to high speed by reversibly changing an amount of magnetic flux of some of a plurality of permanent magnets (referred to as a “variable magnet motor” below). The variable magnet motor is configured by using two types or more of permanent magnets in, for example, an IPM (interior permanent magnet) type motor.
For example, in a permanent magnet motor 21 disclosed in Japanese Patent Laid-Open No. 2010-154676, one first permanent magnet 33 and two second permanent magnets 34 are housed in each permanent magnet housing hole 31a of a rotor 24 as shown in FIG. 2. The two second permanent magnets 34 are located on opposite circumferential sides of the first permanent magnet. The first permanent magnet 33 is a high-coercivity permanent magnet, and has a fixed amount of magnetic flux. The second permanent magnets 34 are low-coercivity permanent magnets, and have variable amounts of magnetic flux.
A rotor 1 disclosed in Japanese Patent Laid-Open No. 2010-4673 includes a rotor core 2, and permanent magnets 3 and 4. The permanent magnets 3 and 4 are arranged in the rotor core 2. The product of a coercivity and a thickness in a magnetization direction of the permanent magnet 3 is smaller than that of the permanent magnet 4. A pair of permanent magnets 4 are located on opposite circumferential sides of each permanent magnet 3. A pair of magnetic barriers (flux barriers) are arranged between the pair of permanent magnets 4 and an outer circumferential surface of the rotor core 2, respectively. No magnetic barrier is arranged between the permanent magnet 3 and the outer circumferential surface of the rotor core 2.
Generally, a flux barrier is arranged around a permanent magnet in a rotor core of the IPM type motor. This is to increase an amount of magnetic flux contributing to torque by causing magnetic flux to easily flow between a stationary portion and a rotating portion. As a result, torque and efficiency in the motor can be improved.
Although the flux barrier is also provided in the rotor 1 disclosed in Japanese Patent Laid-Open No. 2010-4673, the flux barrier is provided for the purpose of efficiently changing an amount of magnetic flux of the permanent magnet 3. However, when the flux barriers are arranged between the permanent magnets 4 with a large amount of magnetic flux and the outer circumference of the rotor 1, the efficiency of the motor is decreased.